In a magnetic disk drive, a magnetic head held by an actuator mechanism is positioned over a predetermined track on a rotating magnetic disk and reads or writes data. On the magnetic disk, an area where data can be recorded is defined. When the magnetic disk drive stops operating, the magnetic disk is moved for data protection to a predetermined position outside the area by the actuator mechanism. While the magnetic disk drive is stopped, the magnetic head is held at the predetermined position.
The actuator mechanism is driven by a voice coil motor (VCM). The VCM includes a coil attached to the actuator and a magnet disposed to face the coil. The actuator is fixed to a pivot shaft. When the coil is energized, a dielectric force generated in a portion facing the magnet of the coil swings the actuator about the pivot shaft.
The dielectric force, however, includes force components which are not required to swing the actuator about the pivot shaft because of, for example, an effect of the shape of the coil. Such force components excite, for example, vibration mode in a direction of bending a coil fixing section (coil support) of the actuator (i.e. bending mode) or in a direction of twisting the coil support (i.e. torsion mode). This deteriorates the positioning performance of the actuator mechanism.
The coil and magnet to be included in an actuator mechanism are therefore determined such that they can provide an adequate driving force to allow the magnetic disk drive to fully display its performance and also such that they are shaped to reduce the unwanted force components.
In Japanese Patent Publication No. 2005-327407 (“patent document 1”), an actuator mechanism including a circular coil and a magnet having a concave outer circumference is disclosed.
In recent years, magnetic disk drives have started being widely used not only in the field of storage devices for computers, but also in the field of home appliances including, for example, DVD recorders incorporating a magnetic disk drive. In the field of home appliances, performance requirements for magnetic disk drives are not so severe as in the field of applications to computers. Instead, it is demanded that their prices be held low.
Therefore, in designing magnetic disk drives for use in home appliances, it is necessary to reduce the costs of parts and assembly while securing minimum required functions, for example, by reducing the amount of materials to be used or the number of parts to be used. The access time of a hard disk apparatus for use in a home appliance, for example, can be slower than that of a hard disk apparatus for use in a computer, so that, for the hard disk apparatus for use in a home appliance, the volume of magnet to be included in a voice coil motor (VCM) for driving an actuator can be reduced for a cost reduction.
Compared with a magnetic disk drive using a related-art longitudinal magnetic recording system, a magnetic disk drive using a perpendicular magnetic recording system offers a dramatically enhanced magnetic recording density resulting in an improved data transfer rate. Since the data transfer rate is a major factor in determining the performance of a magnetic disk drive, such an improved data transfer rate can make up for the performance deterioration attributable to the slower access time.
Generally, the coil included in an actuator mechanism is approximately trapezoidally shaped having an arced bottom. The actuator is driven by the dielectric force generated, when the coil is energized, on both sides of the approximately trapezoidal shape. The coil is made by winding wire, to which a predetermined tensile force is applied, on an approximately trapezoidal winding core which is to make up the inner circumference of the coil. When this is done, the tensile force applied to the wire portion wound around the top of the approximately trapezoidal shape becomes larger than the tensile force applied to other portions of the wire. This can cause individual coils to be finished into non-uniform shapes and results in a reduced yield in coil production.
The actuator mechanism disclosed in the patent document 1 uses an approximately circular coil. Since the inner circumference of the coil is circular, the wire being wound in the coil production process is subjected to a uniform tensile force. This can prevent the above-described yield reduction and can eventually contribute toward reducing the cost of the coil as a component of the actuator mechanism.
Patent document 1 discloses a magnet having a concave outer circumference for use in combination with the approximately circular coil. The circular coil that generates the dielectric force to drive the actuator has no straight linear portion, so that the dielectric force generated by the circular coil includes unwanted force components which excite, for example, vibration mode in a direction of bending the coil support of the actuator or in a direction of twisting the coil support as described above. Furthermore, depending on the position of the actuator, the position and shape of the effective coil portion that generates, using the magnetic field of the magnet, the dielectric force for driving the actuator vary. Hence, the unwanted force components vary depending on the position of the actuator. The magnet having a concave outer circumference is designed to adjust the shape of the effective portion of the magnet depending on the position of the actuator so as to suppress, as much as possible, the generation of the unwanted force components.
Because the concave outer circumference of the magnet is special, it is necessary, in the magnet production process, to shape the magnet one by one by punching using a die. This complicates the magnet production process and therefore may pose a cost disadvantage.